A protein kinase inhibitor is a type of enzyme inhibitor that specifically blocks the action of one or more protein kinases. Protein kinases are enzymes that add a phosphate (PO4) group to a protein or other organic molecule, usually on the serine, threonine, or tyrosine amino acid. Hence, protein kinase inhibitors can be subdivided or characterized by the amino acids whose phosphorylation is inhibited: most kinases act on both serine and threonine, the tyrosine kinases act on tyrosine, and a number (dual-specificity kinases) act on all three. There are also protein kinases that phosphorylate other amino acids, including histidine kinases that phosphorylate histidine residues. Phosphorylation is a necessary step in some cancers and inflammatory diseases. Inhibiting the protein kinases can treat these diseases and protein kinase inhibitors are used as drugs. Literature on the use of kinases inhibitors in drug discovery includes “Targeting Protein Kinases for Cancer Therapy” by Mathew D. J et al, Publisher J. Wiley 2010 and also “Protein Kinases as Drug targets” Klebl et al. J. Wiley 2011.